Each gold nucleus contains 197 protons and neutrons bound together by nuclear forces. When these balls of nuclei move at nearly the speed of light, the effects of special relativity cause their lengths to contract along their axis of motion. Instead of spheres, they become like pancakes.

When these pancakes smash into each other, a vast amount of energy is deposited into a tiny region in space, one millionth of one billionth of a meter across. This tiny speck of matter is as hot and dense as the universe was at one micro-second after the Big Bang.

Under these conditions, the protons and neutrons melt into their sub components called quarks and gluons and a Quark-Gluon-Plasma (QGP) forms. This cocktail of the fundamental constituents of matter quickly expands and cools. Within just 10-22 seconds, the quarks and gluons in the tiny speck of QGP freeze back into composite particles which stream away from the collision zone and are detected in massive particle detectors.

The figure below, shows a schematic diagram of the collision evolution.